In short. A single-phase servo stabiliser serves single-phase loads such as air conditioners, medical and lab equipment, workshops and machines. Size it for the running kVA plus starting surges, check the input range against the supply, and install with the correct neutral and earth.
Typical uses
- Air conditioners and refrigeration with compressors.
- Diagnostic and lab equipment.
- CNC and small machine tools on single-phase supply.
- Commercial shops, studios and small offices.
Sizing
kVA = V × I ÷ 1000
- Add loads that run together; apply a realistic diversity.
- Check the starting current of motors and compressors.
- Add margin for growth.
- Choose the next standard rating above.
See kVA guide.
Input range and tolerance
Select a model whose range covers the lowest and highest voltage measured, with the load at its worst. A wide-range model costs more but avoids shutdowns at extremes. See wide-range stabilisers.
Installation
- Connect the neutral through, as per manufacturer wiring.
- Provide a proper earth.
- Protect upstream with a correctly rated breaker.
- Ventilate the unit.
- Provide a bypass for critical loads.
Generator use
On a generator, frequency and voltage can vary. Check the stabiliser’s input range and frequency tolerance. Avoid running a stabiliser on a lightly loaded generator that hunts. See DG and stabiliser.
Illustrative example
A diagnostic centre has a 5 kVA scanner supply with voltage between 170 and 260 V on a 230 V nominal supply. A single-phase servo stabiliser with a wide input range holds the output steady. The kVA includes margin for the scanner’s inrush and a spare for a future unit.
Why air conditioners and compressors need extra care
A compressor motor draws several times its running current at start. During that instant, the supply voltage sags because of the extra current through the supply impedance. If the stabiliser or supply cannot hold up, the motor fails to start and may trip on overload. The cure is adequate stabiliser capacity and a supply that is not further weakened by long, thin wiring.
Sizing example (illustrative)
| Item | Value |
|---|---|
| 1.5 ton split AC running | About 1.6 kVA |
| Starting surge | Several times running current for a short time |
| Other loads | Lighting and electronics, about 1 kVA |
| Chosen stabiliser | Rated well above running total, with surge capability |
The right approach is to take the maker’s starting current figure, check the stabiliser’s overload rating, and choose accordingly.
Wiring and supply checks before blaming the stabiliser
- Measure voltage at the stabiliser input with and without the load running.
- Check cable size and length from the meter. A long, thin run loses voltage under load.
- Look for loose or heated terminals.
- Confirm the earth connection.
Common mistakes
- One stabiliser for a whole building without allowing for simultaneous starts.
- Sizing to running power only.
- Placing the stabiliser in a closed cupboard with no air flow.
- Using a relay type where frequent compressor starts will wear contacts. See servo vs relay.
Medical and lab equipment
Diagnostic equipment may have its own tolerance and manufacturer requirements. Check what the equipment maker specifies for supply voltage, tolerance and earthing, and consider isolation or a UPS where the maker requires it.
Summer demand in Hyderabad
Peak air-conditioning demand in summer often coincides with the lowest supply voltage in some areas. Measure at the hottest and most loaded time of day, because that is when the stabiliser will be tested hardest.
Verification after installation
With the stabiliser installed, start the largest motor and watch output voltage on a meter. If the output dips far below its normal band, review capacity and wiring. Keep that reading as a baseline.
The master engineering question
Does the chosen rating cover the worst running and starting case at the lowest supply voltage?
What to send us
Send load nameplates and voltage readings. Send your application details.
Questions engineers ask
Can one stabiliser serve many loads?
Yes, if total demand and inrush are within rating.
Why does my AC trip with a stabiliser?
Capacity may be too low for starting, or input range may be insufficient.
Is a single-phase unit enough for three-phase loads?
No. Use a three-phase unit.
Can I add a UPS behind it?
Yes, for loads needing no-break supply.
Values are illustrative. Use manufacturer ratings.